1. Field of the Invention
The present invention relates to a heat-resistant polyimide film, and more particularly, it relates to a polyimide film having improved adhesive properties which is produced by forming a film from a raw material incorporated with a finely divided inorganic powder that forms minute projections on the film surface and subsequently subjecting the film to corona discharge treatment.
2. Description of the Prior Art
It is known that polyimide film has good heat resistance, low temperature resistance, chemical resistance, electrical insulation properties, and mechanical strength, and other good properties. It finds general use as electrical insulation film, heat insulation film, and base film of flexible printed circuit board. A polyimide film as the base film of flexible printed circuit board is bonded to a copper foil with an adhesive. There is often an instance where the bonding is poor on account of the chemical structure and high chemical (solvent) resistance of a polyimide film. At present, this drawback is mitigated by subjecting a polyimide film to surface treatment such as alkali treatment, corona discharge treatment, and sandblasting.
The conventional alkali treatment or sandblasting encounters some problems because it has to be performed separately after the film-making step and requires the additional steps of washing and drying. All this reduces productivity, increases production cost, and impairs stable production. In contrast, corona discharge treatment has an advantage over the above-mentioned two methods of treatment. It has long been used to improve the printability of paper and plastics moldings, and it can be easily performed in-line on the film-making machine because it needs only a simple apparatus.
The present inventors investigated corona discharge treatment as a means to improve the adhesive properties of polyimide film. As the result, it was found that corona discharge treatment is slightly effective in the case of polyimide film with smooth surface produced by a casting method from a raw material containing no inorganic fine particles. However, the adhesive strength improved by corona discharge treatment is generally low and uneven on the film surface. It was also found that the corona-treated film causes blocking when it is wound into a roll and that the blocking is accompanied with organic decomposition products which presumably result from the corona discharge treatment. All this indicates that the corona discharge treatment is not completely practical.